The Pennsylvania State University. The Graduate School. Department of Electrical Engineering EVALUATION OF QUARTER-WAVE VERTICAL MONOPOLE ANTENNAS

Transcription

1 The Pennsylvania State University The Graduate School Department of Electrical Engineering EVALUATION OF QUARTER-WAVE VERTICAL MONOPOLE ANTENNAS WITH ELEVATED RADIALS A Thesis in Electrical Engineering by Arpan Ghosh 2009 Arpan Ghosh Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science May 2009

2 ii The thesis of Arpan Ghosh was reviewed and approved* by the following: James K. Breakall Professor of Electrical Engineering Thesis Advisor Lynn Carpenter Associate Professor of Electrical Engineering Ken Jenkins Professor of Electrical Engineering Head of the Department of Electrical Engineering *Signatures are on file in the Graduate School

3 iii ABSTRACT Most medium-wave broadcast stations use a ground-mounted vertical monopole antenna with buried radials for transmission. About twenty years ago, it was argued that using elevated radials would provide superior performance. The Lawrence Livermore National Laboratory conducted research in the area, modeling an elevated radial system using the Numerical Electromagnetics Code (NEC), over a variety of ground planes at several frequencies throughout the HF band. This is essentially a continuation of that research, using a newer version of NEC, as well as the more recent antenna modeling programs FEKO and XFDTD. There has been much debate in the antenna community that NEC does not provide accurate enough results to be conclusive. This experiment was designed to verify that the results obtained twenty years ago still hold up, and are indeed valid.

4 iv TABLE OF CONTENTS List of Figures...vi Acknowledgements...vii Chapter 1 Background Elevated Radial Systems Debate and the Need for Further Study...3 Chapter 2 Software Comparison Ground System Setup Modeling Software GNEC FEKO XFDTD...7 Chapter 3 Models and Simulation NEC FEKO XFDTD...16 Chapter 4 Results and Conclusions LLNL Results GNEC and FEKO...19

5 v 4.3 XFDTD Conclusions...21 Bibliography...22 Appendix A: NEC-2 Results for Elevated Radial Systems with 4 Radials at 8 MHz...23 Appendix B: NEC-4 Results for Elevated Radials Systems with 4 Radials...25 NEC-4 Results for Elevated Radials Systems with 30 Radials...31 Appendix C: FEKO Results for Elevated Radials Systems with 4 Radials...37 FEKO Results for Elevated Radials Systems with 30 Radials...43 Appendix D: XFDTD Results for Elevated Radials Systems with 4 Radials...49 XFDTD Results for Elevated Radials Systems with 30 Radials...55

6 vi List of Figures Figure 1-1. Setup of 32-Foot Monopole with 4 32-Foot Elevated Radials, Supported by Guy Ropes and Masts...2 Figure 3-1. NEC Model of Antenna Structure with 4 Radials...9 Figure 3-2 NEC Model of Antenna Structure with 30 Radials...10 Figure 3-3 3D NEC Model of Antenna Structure with 4 Radials...11 Figure 3-4 FEKO Model of Antenna Structure with 4 Radials...13 Figure 3-5 FEKO Model of Antenna Structure with 30 Radials...14 Figure 3-6 Segmentation in FEKO...15 Figure 3-7 XFDTD Model of Antenna Structure with 4 Radials...16 Figure 3-8 XFDTD Mesh of Antenna Structure with 30 Radials...18

7 vii ACKNOWLEDGEMENTS I would first of all like to thank my family for all their years of support, and for working so hard to ensure I could get a good education. Thank you to my father especially, who has always given me the freedom to pursue my own goals, and instilled in me a love of engineering. I would also like to thank Dr. Breakall for giving me the opportunity to work on this project, and for shaping the latter half of my graduate school life. His constant guidance and support has aided me immensely in the completion of this project. Thank you to Dr. Carpenter for volunteering his time to be on my thesis committee. It was only an hour of time, but had a very big impact on my future. Lastly, I want to thank Kyle Labowski, a fellow graduate student in my lab, for being a second pair of eyes, and spotting certain mistakes that I may have overlooked. Without his help, I might still be working on this project right now, without any useful results.

8 1 Chapter 1 Background 1.1 Elevated Radial Systems For many years, medium-wave broadcast stations have used a ground-mounted vertical monopole system for transmission. This has been the standard for commercial AM stations, as mandated by the FCC. The system generally consists of 120 or more radials buried under ground, desirably of the same length as the radiating monopole. However, computer studies conducted in the mid-1980 s indicated that an elevated radial system might be a much better option, providing much better field strength. In 1988, an experiment was conducted by the Lawrence Livermore National Laboratory (LLNL) to compare the performance characteristics of a vertical monopole antenna used with different ground systems. Some of these systems were constructed and tested in the field, though all of them were modeled and simulated using the Numerical Electromagnetic Code (NEC). NEC was predominantly used in all such studies at the time. The field test proved to be inconclusive, and it was hard to tell whether elevated radials really were an improvement over buried radials or not. According to the report on the subject, weather conditions and equipment problems were most likely to blame for that, but the actual concept of using elevated radials was not faulty. As such, computer studies were used to predict what the results would be.

9 Computer simulations were performed at various frequencies in the HF band, 2 ranging from 2 MHz to 24 MHz. For the purposes of this study, only one case is of interest, at 8 MHz. This is where the structure would be closest to quarter-wave resonance. Figure 1-1. Setup of 32-foot monopole and 4 32-foot radials, supported by guy ropes and masts

10 1.2 Debate and the Need for Further Study 3 There was a lot of debate however, that NEC might not be the most accurate model for antenna performance, and that the results obtained do no reflect an actual elevated radial system well enough. In the past twenty years, many newer and more sophisticated antenna modeling programs have been developed. Notable among them are FEKO and XFDTD. Both of these programs were used, in addition to a newer version of the NEC code, to validate the results of previous experiments, and to verify that elevated radials do provide better field strength than ground systems. Unfortunately, no field patterns from ground mounted systems were available for comparison, but if the results match those of the LLNL experiment twenty years ago, that should offer some conclusive evidence. The data from all three programs is included at the end of the report.

11 Chapter 2 4 Software Comparison 2.1 Ground System Setup Two different system configurations were used in this experiment, at different frequencies. Both involved a quarter-wave monopole as the radiating antenna, at a certain height above ground. The first system had 4 radials, also quarter-wave, attached to the base of the antenna, while the second system used 30 radials. The monopole radiator was made to be significantly thicker than the radials. Tests were also conducted at three frequencies in the HF band: 1.8 MHz, 3.8 MHz and 7.2 MHz. Unlike the LLNL experiment, only one type of ground was considered here, with a conductivity of Siemens per meter, and a dielectric constant of 15. The system was tested at four different heights above the ground plane, the lowest elevation being half an inch, and the highest 20 feet. 2.2 Modeling Software As mentioned previously, three different antenna modeling programs were used to verify the results of the previous experiment. Two of them, GNEC and FEKO, are fairly similar to each other in terms of how they operate, while the third, XFDTD, is the newest of the three, and uses a different principle entirely.

12 NEC The first of these is a slightly updated version of the Numerical Electromagnetics Code, or NEC. The old experiment used the NEC-2 version, whereas the NEC-4 version was used here. The differences between these versions are slight, and are mainly aesthetic. NEC uses the method of moments, which is a numerical method of solving linear partial differential equations that have been formulated as integral equations. This method is used for modeling antennas, and a variety of other metallic structures. Radiating systems can be modeled in free space or over ground. The system can be isolated from the ground plane, or penetrate between media. To excite the system, one can use a voltage source placed at a point, or an incident plane wave. Different outputs can also be selected: currents, radiation patterns, gain patterns, and even impedance characteristics. If an antenna is modeled over a ground plane, the ground can either be perfectly conducting, or lossy. For a lossy ground, relative permittivity and conductivity must be specified. NEC also has two different types of finite ground. A reflection coefficient approximation is the easiest, but not very accurate. There is also a Sommerfeld/Norton ground, which was used for this particular experiment.

13 2.2.2 FEKO 6 The next program, FEKO, is similar to NEC in that it also utilizes the method of moments for its calculations. However, FEKO is slightly newer, and offers many more options than NEC does. There are two main applications that comprise FEKO: CADFEKO and EDITFEKO. CADFEKO is a 3D modeling tool that can be used to create very complex geometries. It is typically used in the simulation of wire, horn and reflectors antennas, but it can also be used to create 3D models of vehicles, and even people. EDITFEKO on the other hand, is text based, much like NEC, though there is a larger array of commands available for modeling. EDITEFEKO has all the features of CADFEKO, minus the 3D aspect. However, it offers more variety than NEC when it comes to ground, excitation and output options.

14 2.2.3 XFDTD 7 The third, and most recent, program used was XFDTD. Unlike the previous programs, XFDTD does not use the moment method; instead, it uses the finite difference time domain (FDTD) method, from which it derives its name. The biggest difference between this and the previous software is that NEC and FEKO work in the frequency domain, whereas XFDTD, as the name suggests, works in the time domain. This method works by discretizing the time-dependent Maxwell s equations, and solving the resulting finite difference equations. The advantage of working in the time domain is that a broad spectrum of frequencies can be examined in a single run. The FDTD method also calculates fields as they evolve in time, so the electromagnetic fields can be displayed as they propagate in real time. The downside is that, compared to NEC and FEKO, XFDTD requires a slightly longer computation time, as it requires a fairly fine grid to model a particular structure. Whereas NEC and FEKO look at an infinite space, XFDTD examines a finite boundary, so absorbing boundary conditions must be set. It is, like CADFEKO, a 3D modeling tool, and has many of the same features. Once again, different excitations can be set, and the user can define their own materials with different dielectric and magnetic properties.

15 Chapter 3 8 Models and Simulation In the previous work done on this subject, not much information was provided about the model and how it was created. However, since NEC has a very simple interface, it was not very difficult to figure it out. The model consisted of 5 wires, each 32 feet in length; one of them was the monopole antenna, and the other 4 were the radials. The antenna length was kept constant throughout all the frequencies tested. For this experiment, the antenna length is always a quarter of the wavelength being observed. 3.1 NEC Figures 3-1 and 3-2 show the NEC models for both systems. Each wire is composed of 10 segments, for the sake of simplicity, and the feed point is placed at the first segment of the main antenna, which is essentially the intersection of all the wires. The antenna structure is modeled over an imperfect ground plane, with loss parameters as specified earlier. This is a very basic model, with a simple grid-like ground, and very rudimentarylooking thin wires. It should be noted that though the ground appears as grid in the model, the program itself does not treat it that way; it is seen as an infinite plate. One can view the segmentation of the wires if desired, and the model can be rotated or zoomed for closer inspection. The position of the voltage source can also be shown, as well as the currents on the segments.

16 A 3D option is also available, as seen in Figure 3-3. Once again, the model is very simplistic, and somehow, the ground does not appear in the image. 9 Figure 3-1. NEC Model of Antenna Structure with 4 Radials

17 Figure 3-2. NEC Model of Antenna Structure with 30 Radials 10

18 Figure D NEC Model of Antenna Structure with 4 Radials 11

19 3.2 FEKO 12 In the FEKO models, shown in Figures 3-4 and 3-5, we see a little more detail. The segmentation is a little finer. For EDITFEKO, the segment size is chosen to be about 0.6 feet. This is because previous work with FEKO has shown that smaller segments sizes provide more accurate results. FEKO generates a 3D model by default, so the antenna appears as a collection of small cylinders, clearly marking the segmentation. Like NEC, the source can also be viewed, but in addition to showing its position, FEKO also gives its direction. The ground plane is also more defined than NEC, usually appearing as a flat plate rather than a grid. Using a dielectric ground with FEKO did not prove to be very accurate, so instead the Green s Function option was used, which yielded much better results. For the purposes of this experiment, the Green s Function ground plane was given a depth of 100 feet, leading to the box-like structure seen in Figure 3-5.

20 Figure 3-4. FEKO Model of Antenna Structure with 4 Radials 13

21 Figure 3-5. FEKO Model of Antenna Structure with 30 Radials 14

22 15 Figure 3-6 shows a closer look at the segmentation of the model. The red dot at the bottom of the antenna represents the location of the source. There are also a lot more options in this model. It is possible to rotate and zoom, and see the currents, but one can also see the near and far fields as they radiate from the antenna. Figure 3-6. Segmentation in FEKO

23 3.3 XFDTD 16 The XFDTD model is, in some ways, a combination of the two previous models. The antenna system is shown as a group of wires, though the ground plane is still a solid piece; again, a 3D option is available. One convenient aspect of XFDTD is that it shows 3 different views, from 3 different planes (XY, YZ and XZ), as seen in figure 3-7. This makes it easier to view the whole antenna without having to rotate the image, though a rotation tool is available. Figure 3-7. XFDTD Model of Antenna Structure with 4 Radials

24 17 As mentioned earlier, users can define their own materials with different dielectric properties. This is necessary to define the ground plane in XFDTD because, unlike GNEC or FEKO, there is no ground option. The user must create a plate or a rectangular box, and define its material characteristics. This accounts for the different colors seen in Figure 3-7. The antenna is a perfect conductor (white), while the ground is a user-defined material (pink). Once the model has been defined as such, a mesh must be generated, much like FEKO. However, while FEKO s mesh is a part of the model itself, XFDTD defines it in a separate area, shown in Figure 3-8. Here, the user can determine the mesh size and padding. It even allows different areas to be meshed at different sizes. Another very convenient feature of XFDTD is that it shows the amount of memory required for a particular mesh size, which can help in keeping track of the problem size.

25 Figure 3-8. XFDTD Mesh of Antenna Structure with 30 Radials 18

26 Chapter 4 19 Results and Conclusions 4.1 LLNL Results Appendix A shows the results of the previous experiment, as performed with NEC-2. It can be seen that as the height of the structure above the ground was increased, the results were much more desirable. Thus, the best performance was obtained at a height of 5m above ground. 4.2 GNEC and FEKO Appendices B, C and D show the results of the current experiment, as performed with NEC, FEKO and XFDTD, respectively. For the 4 radials case, the NEC results match up with the simulation results of the previous experiment, as they should. There are minor differences, as the models are not exactly the same, but the patterns do follow the same trend. FEKO matches up with NEC quite well, for the most part. The notable exception here is the very first configuration. While NEC suggests field strength well below 0 db, FEKO seems to be significantly higher. Despite repeated runs with changes to the FEKO model, the result appears to be the same. The reason for this disparity is not known, but it seems to be the only one present. Barring that, FEKO seems to agree with NEC quite well. With 30 radials, the results also look quite good. It can be noted that there are some more noticeable discrepancies between GNEC and FEKO at the lower frequency,

27 20 but other than that, it s quite comparable to the 4 radials case. When actually constructing the system, it might be cheaper to only use 4 radials, but in either case, the results are good. 4.3 XFDTD XFDTD provides the most interesting results of the three programs. Looking at the results, the patterns once again follow the same exact trend. However, instead of closed loops, as would be expected over a lossy ground, the XFDTD results display open ones, much like free space. As mentioned earlier, XFDTD is a fairly recent program, and the most current version was used for this experiment. Despite many different configurations and changes in parameters, the models could not be made to agree exactly with NEC and FEKO. It seems that much more work will need to be done on XFDTD to understand fully how the modeling and output data can be managed. For now, the results can be considered inconclusive, but, it should be noted that while the patterns are different, the overall shape appears to be quite similar. Like the other two programs, the pattern generated by XFDTD tends to increase with increasing height, and flattens out a bit at higher frequencies.

28 4.4 Conclusions 21 In the past twenty years, there has been much debate over whether an elevated radial system is actually better than a ground-mounted system in the long-run. Experiments were conducted in 1988 at the Lawrence Livermore National Laboratory, modeled with the Numerical Electromagnetic Code (NEC), and they seemed to support this theory. In recent years, those results have been disputed over time. The purpose here was to try and replicate those results with newer, more modern software, and to validate the previous findings. The programs used were a newer version of NEC, as well as FEKO and XFDTD. The first two programs both use the method of moments in their modeling, whereas the third uses the finite-difference time domain method. As can be seen from the radiation patterns, GNEC and FEKO seem to match up quite well, and also suggest that an elevated system works quite well. XFDTD on the other hand, is not quite as conclusive with its results. This can be attributed to inexperience with using the program. Future work done on the subject should yield something more concrete. Overall though, it seems that the findings at LLNL still hold up quite well today.

29 Bibliography Constantine A. Balanis, Antenna Theory Analysis and Design, Jon Wiley & Sons, Inc., New York, Warren L. Stutzman and Gary A. Thiele, Antenna Theory and Design, Jon Wiley and Sons, Inc, New York, Liang Chi Shen and Jin Au Kong, Applied Electromagnetism, PWS Publishing Company, Boston, Al Christman, R. Zeineddin, Roger Radcliff and Jim Breakall, Using Elevated Radials in Conjunction with Deteriorated Buried-Radial Ground Systems, IEEE Transactions on Broadcasting, Vol.39, No.2, June Al Christman, Roger Radcliff, Dick Adler, Jim Breakall and Al Resnick, AM Broadcast Antennas with Elevated Radial Ground Systems, IEEE Transactions on Broadcasting, 1988 Edition. 6. Al Christman and Roger Radcliff, Impedance Stability and Bandwidth Considerations, IEEE Transactions on Broadcasting, Vol.35, No.2, June J.K. Breakall and A.M. Christman, Evaluation of Vertical Monopole Antennas with Elevated Radials, Lawrence Livermore National Laboratory, March 1988.

30 Appendix A 23 NEC-2 Results for Elevated Radial Systems with 4 Radials at 8 MHz A 32 foot monopole with 4 radials buried 2 inches under ground Frequency: 8 MHz A 32 foot monopole with 4 radials elevated 1 meter above ground Frequency: 8 MHz

31 24 A 32 foot monopole with 4 radials elevated 3 meters above ground Frequency: 8 MHz A 32 foot monopole with 4 radials elevated 5 meters above ground Frequency: 8 MHz

32 Appendix B 25 NEC-4 Results for Elevated Radial Systems with 4 Radials Frequency: 1.8 MHz

33 26

34 Frequency: 3.8 MHz 27

35 28

36 Frequency: 7.2 MHz 29

37 30

38 NEC-4 Results for Elevated Radial Systems with 30 Radials 31 Frequency: 1.8 MHz

39 32

40 Frequency: 3.8 MHz 33

41 34

42 Frequency: 7.2 MHz 35

43 36

44 Appendix C 37 FEKO Results for Elevated Radial Systems with 4 Radials Frequency: 1.8 MHz

45 38

46 Frequency: 3.8 MHz 39

47 40

48 Frequency: 7.2 MHz 41

49 FEKO Results for Elevated Radial Systems with 30 Radials 42

50 Frequency: 1.8 MHz 43

51 44

52 Frequency: 3.8 MHz 45

53 46

54 Frequency: 7.2 MHz 47

55 48

56 Appendix D 49 XFDTD Results for Elevated Radial Systems with 4 Radials Frequency: 1.8 MHz A lambda/4 monopole with 4 radials elevated 0.5 inches above ground Frequency: 1.8 MHz A lambda/4 monopole with 4 radials elevated 3 feet above ground Frequency: 1.8 MHz

57 50 A lambda/4 monopole with 4 radials elevated 10 feet above ground Frequency: 1.8 MHz A lambda/4 monopole with 4 radials elevated 20 feet above ground Frequency: 1.8 MHz

58 Frequency: 3.8 MHz 51 A lambda/4 monopole with 4 radials elevated 0.5 inches above ground Frequency: 3.8 MHz A lambda/4 monopole with 4 radials elevated 3 feet above ground Frequency: 3.8 MHz

59 52 A lambda/4 monopole with 4 radials elevated 10 feet above ground Frequency: 3.8 MHz A lambda/4 monopole with 4 radials elevated 20 feet above ground Frequency: 3.8 MHz

60 Frequency: 7.2 MHz 53 A lambda/4 monopole with 4 radials elevated 0.5 inches above ground Frequency: 7.2 MHz A lambda/4 monopole with 4 radials elevated 3 feet above ground Frequency: 7.2 MHz

61 54 A lambda/4 monopole with 4 radials elevated 10 feet above ground Frequency: 7.2 MHz A lambda/4 monopole with 4 radials elevated 20 feet above ground Frequency: 7.2 MHz

62 XFDTD Results for Elevated Radial Systems with 30 Radials 55 Frequency: 1.8 MHz A lambda/4 monopole with 30 radials elevated 0.5 inches above ground Frequency: 1.8 MHz A lambda/4 monopole with 30 radials elevated 3 feet above ground Frequency: 1.8 MHz

63 56 A lambda/4 monopole with 30 radials elevated 10 feet above ground Frequency: 1.8 MHz A lambda/4 monopole with 30 radials elevated 20 feet above ground Frequency: 1.8 MHz

64 Frequency: 3.8 MHz 57 A lambda/4 monopole with 30 radials elevated 0.5 inches above ground Frequency: 3.8 MHz A lambda/4 monopole with 30 radials elevated 3 feet above ground Frequency: 3.8 MHz

65 58 A lambda/4 monopole with 30 radials elevated 10 feet above ground Frequency: 3.8 MHz A lambda/4 monopole with 30 radials elevated 20 feet above ground Frequency: 3.8 MHz

66 Frequency: 7.2 MHz 59 A lambda/4 monopole with 30 radials elevated 0.5 inches above ground Frequency: 7.2 MHz A lambda/4 monopole with 30 radials elevated 3 feet above ground Frequency: 7.2 MHz

67 60 A lambda/4 monopole with 30 radials elevated 10 feet above ground Frequency: 7.2 MHz A lambda/4 monopole with 30 radials elevated 20 feet above ground Frequency: 7.2 MHz